1. Field of the Invention
The present invention relates to a circuit for operating recording nozzles in ink mosaic recorders, and in particular to such a circuit wherein the nozzles have an ink channel which is surrounded by a cylindrical piezoelectric transducer which has a diameter which expands upon the application of a voltage having a polarity opposite to the polarization direction of the transducer and which contracts or narrows upon the application of a voltage having a polarity which is opposite to the direction of the polarization.
2. Description of the Prior Art
A circuit for operating tubular piezoelectric transducers in recording nozzles of an ink mosaic recorder is known from German AS No. 25 48 691, corresponding to U.S. Pat. No. 4,161,670. As disclosed therein, the ejection of a droplet of ink is initiated by expansion of the piezoelectric transducers by the application of a voltage having a polarity which is opposite to the direction of the polarization of the transducers, followed by contraction of the transducers as a result of a reversal of the polarity of the operating voltage. The transducers normally are in the expanded condition which is thus referred to as the rest state. The expansion of the transducers lead to a small quantity of ink being drawn into the nozzle, whereas the narrowing of the transducer results in the ejection of a droplet of ink. In this known circuit, each piezoelectric transducer is assigned a voltage transformer having a primary side which contains a pulse evaluating stage and an amplifier stage. The secondary side inductance of the transformer forms an oscillating circuit in combination with the capacitance of the piezoelectric transducer. Pulsed operation on the primary side of the transformer results in the rising slope of the operating pulse on the secondary side inducing a surge of voltage having a polarity such that the transducer expands. Following the disconnection of the operating pulse, producing a declining slope, a voltage is induced in the opposite direction and triggers a contraction of the transducer. Suitable circuit means insure that the oscillation which occurs on the secondary side is attenuated such that it rapidly decays to substantially zero after a short length of time. An advantage of this circuit is that the voltage range available for the change in diameter of the transducer can be selected over a large number of values. This circuit has the disadvantage, however, that voltages occur, even though only temporarily, having a polarity which is opposite to the direction of polarity of the ceramic material comprising the piezoelectric transducer. Particularly at high voltages this can lead to a depolarization of the piezoelectric material over the course of time.
In order to avoid depolarization of the piezoelectric ceramic material, a circuit disclosed in U.S. patent 3,683,212 maintains the piezoelectric operating element in the rest state free of voltage. In this known circuit, a droplet of ink is ejected by means of a triggering by an operating pulse having a voltage with a polarity in the direction of polarization which is applied to the operating element. The resulting charging leads to a reduction in diameter of the element. At the end of the operating pulse a relatively slow discharge of the element takes place during which the element re-expands to its original diameter, corresponding to the rest state, and draws ink from a reservoir in readiness for the next actuation. Although this known circuit avoids the occurrence of voltages having a polarity which is opposite to the direction of polarization of the transducer element, the circuit requires a voltage source located directly in the control circuit which is assigned to each transducer and which must meet certain requirements such as, for example, a low internal impedence. Moreover, the voltage range of operation is limited because the charging of the transducer is restricted by the ratio of the charging resistor and the discharging resistor values. This circuit also permits ejection of a droplet of ink only in the sequence of a transducer contraction followed by a transducer expansion.